(1) Field of the Invention
The present invention relates to an optical device used for optical communication, and more particularly, to a technique suitable for use in an optical device, such as an optical modulator having an optical waveguide structure.
(2) Description of Related Art
An optical communications device typified by an optical modulator using an electro-optical crystal, such as an LiNbO3 [lithium niobate (LN)] crystal substrate or an LiTaO2 [lithium tantalate (LT)] crystal substrate, is formed by placing electrodes in the vicinity of an optical waveguide after a metal film has been formed over a portion of the crystal substrate and then the film has been thermally diffused, or after a metal film has been patterned and the thus-patterned metal film has been subjected to proton exchange in a benzoic acid.
In the case of, e.g., an intensity modulator, the optical waveguide is formed from an incoming waveguide, an incoming Y-branched waveguide, intensity modulation waveguides (parallel waveguides), an outgoing Y-branched waveguide, and an outgoing waveguide. In the case of a phase modulator, the optical waveguide is formed from an incoming waveguide, phase modulation waveguides (linear waveguides), and an outgoing waveguide. A signal electrode (also called a “hot electrode”) and an earth electrode (also called a “ground electrode”) are provided on the respective modulation waveguides, to thus form a coplanar electrode. When a substrate (a Z-cut substrate) which is cut along the Z-axis direction of a crystal orientation is used, a change in refractive index due to an electric field in the direction Z is utilized. Therefore, the electrode is disposed immediately above the waveguide.
In the case of the intensity modulator, a signal electrode is patterned on one of the modulation waveguides (parallel waveguides), and the ground electrode is patterned on the remaining modulation waveguide. In the case of the phase modulator, the signal electrode is patterned on the modulation waveguide, and the ground electrode is patterned in parallel with the signal electrode. However, in order to prevent the signal electrode and the ground electrode from absorbing the light propagating through the modulation waveguide, a dielectric layer (a buffer layer) is interposed between the signal electrode, the ground electrode and an LN substrate. For instance, SiO2 having a thickness of 0.2 to 1.0 μm is used as the buffer layer.
When such an optical communications device is driven at high speed, the terminal of the signal electrode and that of the earth electrode are connected together by means of a resistor, to thus form a traveling wave electrode. A microwave electric signal is applied to an input side of the electrode. In the case of the intensity modulator, the refraction factors of the two parallel waveguides (tentatively called A, B) are respectively changed by +Δna, −Δnb by means of the electric field. Since a phase difference between the parallel waveguides A, B is also changed, signal light whose intensity has been modulated is output from the outgoing waveguide. Now, the effective refraction factor of the microwave is controlled by changing the cross-sectional profile of the electrode, to thus match the speed of light and that of the microwave, whereby a wide band optical response characteristic can be acquired.
As shown in, e.g., FIG. 9, such an intensity modulator 100-1 and a phase modulator 100-2 are connected in series (in any sequence), and both the intensity modulator and the phase modulator are driven by a microwave having a frequency of f0, so that an optical frequency comb generator is realized. Specifically, when a laser beam of single wavelength (frequency) is input from a light source (a laser diode: LD) 200, an optical output consisting of a plurality of wavelengths can be obtained. The interval of the wavelengths of the optical output corresponds to the frequency f0 of the input electric signal. Therefore, the optical output can be used as a multiwavelength light source for WDM (Wavelength Division Multiplex) optical transmission by setting the frequency f0 to an integral multiple of 12.5 GHz. For example, as shown in FIG. 10, a wavelength filter (bandpass filter) 300 which exhibits a Gaussian filtering characteristic on a wavelength axis is connected in series subsequent to the phase modulator 100-2 and the intensity modulator 100-1. When CW light of single wavelength is input from the light source 200, an optical pulse wave having a frequency f0 can be acquired. Detailed descriptions in relation to the optical frequency comb generator are provided in, e.g., Non-Patent Documents 1, 2 which will be described later.
FIG. 11 is a schematic plan view showing a detailed example configuration of such an optical frequency comb generator. The optical frequency comb generator shown in FIG. 11 comprises, as constituent elements of the intensity modulator 100-1, an incoming waveguide 101, an incoming Y-branched waveguide 102, parallel waveguides 103A, 103B, an outgoing Y-branched waveguide 104, a signal electrode 109 arranged such that a part thereof overlaps one (103A) of the parallel waveguides 103A, 103B, and a ground electrode 110 arranged such that a part thereof overlaps the remaining one (103B) of the parallel waveguides 103A, 103B, all these constituent elements being provided on an LN substrate 100. The optical frequency comb generator further comprises, as constituent elements of the phase modulator 100-2, a phase modulation waveguide 105, a signal electrode 112 arranged such that a part thereof overlaps the phase modulation waveguide 105, and ground electrodes 113, 114 arranged on respective sides of the signal electrode 112 (the phase modulation waveguide 105) in parallel thereto.
In the case of this example, in order to reduce a required drive voltage by means of assuring the longest possible interaction length between light and electricity (the microwave), the phase modulation waveguide 105, the signal electrode 112, and the ground electrodes 113, 114, which are constituent elements of the phase modulator 100-2, assume a folded structure with respect to the longitudinal direction of the LN substrate 100. Outgoing light is output from the same side of the LN substrate 100 where the incoming light is input. FIG. 12 is a fragmentary enlarged cross-sectional view taken along A-A in FIG. 11. As mentioned previously, a dielectric layer (a buffer layer) 116 is interposed between the LN substrate 100, the signal electrode 112, and the ground electrodes 113, 114.
Other related-art techniques pertaining to the optical device using an optical waveguide are proposed in, e.g., Patent Documents 1 to 4 provided below.
The technique described in Patent Document 1 relates to an optical modulator, particularly, a Mach-Zehnder modulator, and is intended for rendering the modulator compact and easy to handle. A plurality of pairs of parallel waveguides of the Mach-Zehnder modulator are provided by folding the parallel waveguides at the end face of a substrate. Thus, the length of the substrate is shortened, thereby imparting the modulator with a compact rectangular shape. The resultant modulator can be made easy to handle, and the substrate can be made less susceptible to damage.
The technique described in Patent Document 2 is intended for providing a high-performance optical modulator which enables exhibition of a high-performance transmission characteristic by controlling wavelength chirping without deforming a modulated waveform of light intensity and which further reduces electrical crosstalk. In the optical modulator that is formed from at least two modulation electrodes and an optical waveguide and utilizes an electro-optical effect, the electrical crosstalk can be sufficiently diminished by arranging the two modulation electrodes in series with the waveguide direction of the optical waveguide.
Patent Document 3 aims at providing an optical waveguide element which produces two output light beams containing essentially no wavelength chirping; which exhibits little wavelength dependency; and which is easily integrated. To this end, a one-by-two Y-branched waveguide is connected to input sides of two parallel intermediate optical waveguides. A two-by-two 3 dB coupler is connected to output sides of the intermediate optical waveguides. Two sets of electrodes are provided on or in the vicinity of the two intermediate optical waveguides for changing the refraction factors of the optical waveguides. A complementary electric signal is applied to the two sets of electrodes. When the optical waveguide element of such a configuration is caused to perform switching or modulator operation, the rates of changes in the refraction factors of the two intermediate optical waveguides can be made equal to each other, with opposite signs. Accordingly, the wavelength chirping of the output light can be reduced to 0 with respect to any output light. Even when the light outputs are merged together, a merged signal of excellent quality which does not cause any disturbances in wavelength can be obtained. Consequently, when light is caused to have propagated through the fiber, no wavelength chirping appears. For this reason, there is yielded an advantage of light being less susceptible to influence of deterioration of a waveform, which would otherwise be caused by dispersion stemming from the fiber.
The technique described in Patent Document 4 relates to a waveguide-type Mach-Zehnder optical interferometer and enables stable operation of the optical interferometer which does not depend on a polarizing direction of incoming light, by virtue of locally controlling birefringence of two single-mode optical waveguides over a specific length by means of operation of an applied control groove.                [Patent Document 1] JP-HEI-5-232417A        [Patent Document 2] JP-HEI-7-64031A        [Patent Document 3] JP-HEI-9-288255A        [Patent Document 4] JP-SHO-63-147145A        [Non-Patent Document 1] Sugiyama et al., “Optical Frequency Comb Generation (2) using a LiNbO3 Modulator,” Autumn Meeting of The Institute of Electronics, Information and Communication Engineers, 2004.        [Non-Patent Document 2] M. Sugiyama et al., “A low drive voltage LiNbO3 phase and intensity integrated modulator for optical frequency comb generation and short pulse generation,” ECOC 2004.        
In order to increase the number of wavelengths output from the above-described optical frequency comb generator or to reduce the width of a pulse at the time of generation of the pulse, the voltage (drive voltage) input to the phase modulator 100-2 must be increased, to thus increase a modulation index. However, since a ceiling is imposed on an output from the electrical amplifier, the number of wavelengths is limited. For this reason, a reduction in the drive voltage (a half-wavelength voltage) Vπ of the phase modulator 100-2 poses a problem. The essential requirement for reducing the drive voltage Vπ is to increase the interaction length to the greatest possible extent. However, a restriction is imposed on the size of the substrate, and hence a limit is imposed on an increase in interaction length. Even if the interaction length has been made long, a modulation index consistent with the thus-increased interaction length cannot be obtained, for reasons of attenuation of the microwave traveling through the signal electrode 105. A limit is also imposed on reduction of the drive voltage Vπ. In this respect, all of the above-described related-art techniques are the same as this technique.